The present invention relates to a procedure and an apparatus for controlling a hydraulic elevator during approach to a landing.
At present, hydraulic elevators with an on-off type hydraulic control system (open system) have the drawback that the length of the creeping distance during approach to a landing essentially varies with the load (oil pressure) and oil temperature (change in viscosity).
In certain operational circumstances, this variation may become excessively large and have a negative effect on the capacity of the elevator.
A long creep distance usually also involves an accelerating rise in the oil temperature and may necessitate extra cooling.
In practice, the variations in the creep distance mean that at a normal operating temperature this distance must be quite long to ensure that during low oil temperature operation (for example during the elevator's first drives in the morning), the elevator will not move past the landing when stopping. When the oil temperature is high, the creep distance usually becomes considerably longer, resulting in a reduced elevator capacity and an increased rate of rise of the oil temperature. The effect of the load means that, for example during up-travel at a given oil temperature, the creep distance for a car with full load is substantially longer than for an empty car.
Methods for correction of the deceleration point to achieve a shorter and more constant creep distance for varying loads and temperatures have been known for a long time. One way to accomplish this is as proposed in U.S. Pat. No. 4,534,452, in which a suitable delay at the next deceleration point is selected before start on the basis of load and temperature information. In this case, the effects upon what happens from the point of starting deceleration, due to changes in oil temperature or load variations (resulting from, for example, variations in guide friction) is not taken into account at all. Additionally, producing the load information requires a weighing device, which is often expensive if it is to give a sufficient accuracy.
U.S. Pat. No. 4,775,031 proposes a control method for hydraulic elevators whereby the speed and position of the elevator car is measured by means of a speed sensing tachometer, which requires space and is expensive.